(C) Copyright 2000 Texas Instruments Incorporated. A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.
1. Technical Field of the Invention
This invention relates to data processing devices, electronic processing and control systems and methods of their manufacture and operation, and particularly relates to memory access schemes of microprocessors optimized for digital signal processing.
2. Background of the Invention
Generally, a microprocessor is a circuit that combines the instruction-handling, arithmetic, and logical operations of a computer on a single semiconductor integrated circuit. Microprocessors can be grouped into two general classes, namely general-purpose microprocessors and special-purpose microprocessors. General-purpose microprocessors are designed to be programmable by the user to perform any of a wide range of tasks, and are therefore often used as the central processing unit (CPU) in equipment such as personal computers. Special-purpose microprocessors, in contrast, are designed to provide performance improvement for specific predetermined arithmetic and logical functions for which the user intends to use the microprocessor. By knowing the primary function of the microprocessor, the designer can structure the microprocessor architecture in such a manner that the performance of the specific function by the special-purpose microprocessor greatly exceeds the performance of the same function by a general-purpose microprocessor regardless of the program implemented by the user.
One such function that can be performed by a special-purpose microprocessor at a greatly improved rate is digital signal processing. Digital signal processing generally involves the representation, transmission, and manipulation of signals, using numerical techniques and a type of special-purpose microprocessor known as a digital signal processor (DSP). Digital signal processing typically requires the manipulation of large volumes of data, and a digital signal processor is optimized to efficiently perform the intensive computation and memory access operations associated with this data manipulation. For example, computations for performing Fast Fourier Transforms (FFTs) and for implementing digital filters consist to a large degree of repetitive operations such as multiply-and-add and multiple-bit-shift. DSPs can be specifically adapted for these repetitive functions, and provide a substantial performance improvement over general-purpose microprocessors in, for example, real-time applications such as image and speech processing.
DSPs are central to the operation of many of today""s electronic products, such as high-speed modems, high-density disk drives, digital cellular phones, complex automotive systems, and video-conferencing equipment. DSPs will enable a wide variety of other digital systems in the future, such as video-phones, network processing, natural speech interfaces, and ultra-high speed modems. The demands placed upon DSPs in these and other applications continue to grow as consumers seek increased performance from their digital products, and as the convergence of the communications, computer and consumer industries creates completely new digital products.
Microprocessor designers have increasingly endeavored to exploit parallelism to improve performance. One parallel architecture that has found application in some modern microprocessors utilizes multiple instruction fetch packets and multiple instruction execution packets with multiple functional units, referred to as a Very Long Instruction Word (VLIW) architecture.
Digital systems designed on a single integrated circuit are referred to as an application specific integrated circuit (ASIC). MegaModules are being used in the design of ASICs to create complex digital systems a single chip. (MegaModule is a trademark of Texas Instruments Incorporated.) Types of MegaModules include SRAMs, FIFOs, register files, RAMs, ROMs, universal asynchronous receiver-transmitters (UARTs), programmable logic arrays and other such logic circuits. MegaModules are usually defined as integrated circuit modules of at least 500 gates in complexity and having a complex ASIC macro function. These MegaModules are predesigned and stored in an ASIC design library. The MegaModules can then be selected by a designer and placed within a certain area on a new IC chip.
Designers have succeeded in increasing the performance of DSPs, and microprocessors in general, by increasing clock speeds, by removing data processing bottlenecks in circuit architecture, by incorporating multiple execution units on a single processor circuit, and by developing optimizing compilers that schedule operations to be executed by the processor in an efficient manner. For example, non-aligned data access is provided on certain microprocessors. Complex instruction set computer (CISC) architectures (Intel, Motorola 68K) have thorough support for non-aligned data accesses; however, reduced instruction set computer (RISC) architectures do not have non-aligned accesses at all. Some RISC architectures allow two data accesses per cycle, but they allow only two aligned accesses. Certain CISC machines now allow doing two memory accesses per cycle as two non-aligned accesses. A reason for this is that the dual access implementations are superscalar implementations that are running code compatible with earlier scalar implementations.
The increasing demands of technology and the marketplace make desirable even further structural and process improvements in processing devices, application systems and methods of operation and manufacture.
An illustrative embodiment of the present invention seeks to provide a microprocessor and a method for accessing memory by a microprocessor that improves digital signal processing performance. Aspects of the invention are specified in the claims.
In an embodiment of the present invention, each .D unit of a DSP can load and store double words (64 bits) at aligned addresses in a two port memory subsystem. The .D units can also access words and double words on any byte boundary. The address generation circuitry in the first .D unit has a first address output connected to the first memory port and a second address output selectively connected to the second memory port. The address generation circuitry can provide two addresses simultaneously to request two aligned data items. For circular buffer accesses near the end of a circular buffer region in the memory subsystem, one address is associated with an end of the circular buffer region and the other address is associated with an opposite end of the circular buffer region. An extraction circuit is connected to the memory subsystem to provide a non-aligned data item extracted from two aligned data items requested by the .D unit, such that a non-aligned access near the end of the circular buffer region provides a non-aligned data item that wraps around to the other end of the circular buffer.
In another embodiment of the present invention, the address generation circuitry in the .D units is operable to form an address for non-aligned double word instructions by combining a base address value and an offset value.
In another embodiment of the invention, one or more additional .D units have similar addressing circuitry for circular buffer access.
In another embodiment of the present invention, two .D units can simultaneously access aligned data items in circular buffer regions of the memory.